1. Field of the Invention
The present invention relates to a hydraulic pump for discharging a fluid by means of a slide piston reciprocating in a cylinder and particularly relates to an improvement for facilitating assembling and, at the same time, heightening discharge efficiency.
2. Description of the Related Art
In general, a hydraulic pump comprises a slide piston reciprocating in a cylinder to compress/expand the volume of a pump chamber in the cylinder, a suction valve for opening an inlet provided in the pump chamber in an expansion stroke of the slide piston, a discharge valve for opening an outlet of the pump chamber in a compression stroke of the slide piston, and an elastic sealing member provided between the cylinder and the slide piston to prevent a fluid in the pump chamber from leaking out of a gap between the slide piston and the cylinder, in which the fluid is discharged by the reciprocating motion of the slide piston.
In such a hydraulic pumps, a pump using a cylinder having a relatively large diameter and using a capped-cylindrical piston as the slide piston is called a piston pump, and a pump using a cylinder having a relatively small diameter and using a round-rod-like plunger as the slide piston is called a plunger pump.
As shown in FIG. 6, in the conventional hydraulic pump, generally, a circumferential groove 2 is formed in the inner circumference of the cylinder 1 so that a ring-like elastic sealing member 3 fitted into the groove 2 is brought into elastic contact with the outer circumference of the slide piston 4 such as a piston, a plunger, or the like, to thereby prevent the fluid in the pump chamber 5 from leaking out of a gap 6 between the cylinder 1 and the slide piston 4.
In the way of providing the elastic sealing member 3 as shown in FIG. 6, however, the elastic sealing member 3 is displaced and deformed in a direction of departing from the pump chamber 5 as represented by the arrow (A), by the pressure of the fluid in the pump chamber 5 to increase the substantial volume of the pump chamber 5 as shown in FIG. 7 when the discharge pressure is high. The increment of the volume of the pump chamber 5 due to the displacement and deformation of such an elastic sealing member 3 forms an escape space for the fluid compressed by the slide piston 4, so that there arises a problem in the loss of the discharge quantity.
The more the stroke X of a movement of the slide piston 4 decreases, the stronger the influence of this problem becomes so that there is a risk of a serious defect disturbing improvement in the discharge efficiency of the pump.
If accuracy in the fitting of the elastic sealing member 3 into the groove 2 is improved or accuracy in the fitting of the slide piston 4 into the cylinder 1 is improved to thereby reduce the displacement and deformation of the elastic sealing member 3, the increase of the volume of the pump chamber due to the displacement and deformation of the elastic sealing member 3 can be reduced. When such fitting accuracy is improved, however, there arises a new problem that the cost of machining increases to thereby bring about the increase in cost or poor assembling property.